Evolutionary psychologists have long argued that many of the physical features heterosexual men are drawn to in women reflect traits that signify female health and fertility status. The basic argument is that our male ancestors developed an attraction to these traits because it enhanced their odds of reproductive success. These mating preferences are thought to have been passed down across generations and still influence what men are attracted to today on some primal level. In a new study just published in the journal Evolution and Human Behavior, researchers examined whether men’s attraction to women with more prominent rear ends might represent one such evolved mating preference.

The researchers hypothesized that the shape of a woman’s buttocks is a meaningful cue to female fertility; however, it is not the overall mass of the buttocks that is important, but rather the degree to which it protrudes from the body in response to curvature of the spine. They argue that women who have a very straight spinal column are at a reproductive disadvantage because, when pregnant, their center of mass shifts forward in a way that puts a lot of pressure on their muscles and lower back, thereby increasing pain and risk of injury. In contrast, women with curving of the lower (i.e., lumbar) vertebrae would be able to maintain a center of mass over their hips during pregnancy, which would yield more stability and reduce the odds of injury. Because the appearance of the buttocks can provide some clue as to the degree of curving or “wedging” of the lumbar spine, the researchers speculated that women with more prominent rear ends resulting from such curvature would be deemed more attractive. They presented two studies supporting this idea.

In the first study, 102 male college students viewed photo morphs of women whose appearance was identical, except for the degree of curvature in the lower back, ranging from 26 degrees to 61 degrees. What they found was that as the degree of lumbar curvature approached the theoretically optimal level (i.e., 45.5 degrees, a figure determined by consulting research in orthopedic medicine), the models were rated more attractive. Attractiveness ratings decreased once lumbar curvature exceeded that level (excessive spinal curving can pose a whole other set of health issues, so it makes sense that any preference for spinal curving would only exist to some degree).

In the second study, researchers sought to determine whether lumbar curving is really what men are picking up on, or if guys just like big butts in general. In order to test this idea, 202 male college students were shown a series of morphs of the female figure that had the same size buttocks, but varied in terms of the underlying reasons contributing to its size. That is, they presented female figures who had larger rear ends due to having larger gluteal muscles vs. more fatty tissue vs. lumbar curving. Again, the buttocks was the same size in each set of morphs, it’s just that the position varied depending upon the underlying cause. As predicted, men showed a preference for female figures that featured lumbar curvature, not figures that generally had larger gluteal mass.

Of course, these data are limited in that they only considered college student participants from one cultural context (the United States) who rated their attraction to digital abstractions of female bodies, not actual women. In the real world, attraction is a complex judgment that takes into account numerous factors beyond the appearance of a single body part. This research does not clarify how much spinal curving contributes to perceptions of female attractiveness relative to other physical and psychological traits.

Nonetheless, these results suggest that men really are drawn to “curvy” women—but that one of the curves they may be subconsciously picking up on is the curvature of a woman’s lower back.